1. Field of the Invention
The present invention generally relates to a heterojunction bipolar transistor (HBT) structure, and more specifically to a directed epitaxial HBT structure having an InGaP tunnel collector layer and/or emitter layer formed on a Gallium Arsenide (GaAs) substrate in a specific direction.
2. The Prior Arts
Generally, a heterojunction bipolar transistor (HBT) is a specific transistor having an emitter and a base formed by different semiconductor materials. Accordingly, a heterojunction is formed between the emitter and the base. One advantage of a HBT (such as an NPN HBT) is that the emitter injection efficiency is greatly increased because it is difficult for the hole current to flow through the valence band barrier (ΔEv) between the base and the emitter. The current gain is thus further enhanced under the higher base dopant concentration. The efficiency of power amplifiers is an important factor when HBTs are used in power amplifiers of hand-held devices. It should be noted that the key factors of a HBT are the knee voltage and the turn-on voltage. Therefore, one of the important issues is how to effectively decrease the knee voltage and the turn-on voltage of a HBT.
In the prior arts, a HBT is formed on the GaAs substrate that is formed by a (100) face or a (100) face towards (110) face with an angle of inclination of about 2°. Preferably, the HBT base layer is formed by a GaAs semiconductor material, and the emitter and the tunnel collector are formed by InGaP. Since InGaP is formed on the GaAs substrate by a (100) face towards a (110) face at an angle of inclination of about 2°, it is difficult for indium and gallium to be arranged in a <111> direction in the high degree of order, thereby resulting in the electron affinity of InGaP to be smaller or far smaller than that of GaAs. Thus, the discontinuity exists between the conduction bands of the base-emitter junction and the base-tunnel collector junction. As such, the HBT has a high turn-on voltage. Moreover, the collector current blocking effect is created, and the knee voltage is increased such that the power added efficiency (PAE) of the HBT is decreased.
Therefore, it is necessary for a HBT structure grown on a different inclination face to have the emitter layer and/or the tunnel collector layer, generally formed by InGaP or InGaAsP, a higher electron affinity or a smaller bandgap. The discontinuity between the conduction bands of the base-emitter and base-tunnel collector junctions is then decreased so as to lower the turn-on voltage and the offset voltage of the HBT, thereby improving the collector current blocking effect, decreasing the knee voltage, greatly increasing the PAE of the HBT power amplifier and enhancing the overall electrical performance.